DOPPLER EFFECT CRIME PREVENTION DEVICE

摘要

PURPOSE:To prevent malfunction due to the variance in a component characteristic and a temperature characteristic of the component by processing the number of times of quadrant transition digitally. CONSTITUTION:An ultrasonic wave radiated by a transmitter 1 is reflected by a body and received by a receiver 2, whose receive signal Ein is frequency-converted by a converter circuit 20. A quadrant signal generating circuit 27 sends quadrant signals from output terminals I -IV corresponding to the respective quadrants of the rotary vector of the receive signal Ein and a quadrant transition direction detecting circuit 31 outputs an L- or H- level digital signal according to the rotational vector of the signal Ein moves to another quadrant counterclockwise or clockwise. The output of a threshold value circuit 32 falls to the level L at the time of a change in quadrant regardless of whether the output of the detecting circuit 31 is at the level H or L. When the output of the threshold value circuit 32 falls to the level L, the clock signal outputted by a clock generating circuit 33 is counted by a comparing circuit 34 and a decision circuit 33 outputs an alarm signal when the counted value of the comparing circuit 34 attains to a set value.